This invention is drawn to a method for producing a catalyst used in reforming and aromatization reactions. It may be used in processes for producing reformates from naphtha streams or alternatively the process of use may entail production of aromatics, such as benzenes or alkyl benzenes, from a feedstock purer than straight run naphtha.
Since the advent of higher compression automobile and aircraft gasoline engines in the late 1930's and 40's, the demand for higher octane gasoline has continually risen. For the past many years, this octane requirement has been supplied by the addition of various organo lead compounds, notably tetraethyllead (TEL), or other similar compounds to mixtures of various hydrocarbons. However, because of the widespread use of catalytic converters for the removal of various undesirable components from the exhaust gases of automobiles (which converters are poisoned by the use of lead in gasoline), other methods of improving motor gasoline octane have become more important. One such method of improving the octane of straight run gasoline fractions is via the use of catalytic reforming.
Catalytic reforming is a commonly practiced process in the petroleum industry. It refers to the treatment of various naphtha fractions to improve their octane rating via their conversion to aromatics. The more important hydrocarbon reactions occurring during the reforming operation include the dehydrocyclization of linear alkanes to aromatics, the dehydrogenation of cycloalkanes to aromatics, and the dehydroisomerization of alkylcyclopentanes to aromatics. A number of other reactions also occur, such as the isomerization of paraffins and the hydrocracking of various hydrocarbons to produce lighter gaseous products. Hydrocracking reactions are generally to be minimized during reforming in that they decrease the yield of the more valuable aromatics and produce hydrocarbons of lower economic value such as methane, ethane and propane.
The production of benzene or alkylbenzenes from linear or branched alkanes is an important process in the chemical industry. Benzene, toluene and the various xylenes form the bases for many polymerization processes. Yield of products and selectivity to the proper products are the major concerns in processes for producing these commodity chemicals. Major by products in the dehydrocyclization of alkanes include those also found in the reforming processes discussed above, e.g., light gases such as methane, ethane and propane.
Catalysts which are suitable for reforming processes must possess a wide variety of chemical and physical characteristics. The catalyst must be able to produce highly aromatic liquid products in high yields. In reforming, the aromatic hydrocarbons must be produced in concentrations suitable for blending to high octane motor fuels. The catalyst should produce low yields of lighter gaseous hydrocarbons. The catalyst should have high activity and should be regnerable with relative ease as time goes on. The catalyst should be fairly strong, i.e., possess good crush strength, and have a high attrition resistance. Thus, the catalyst may be loaded into reaction vessels with a minimal loss of catalyst macrostructure due to physical breakage. The catalyst should be of a form which may be cheaply manufactured.
Catalysts containing platinum, with or without the addition of other promoter metals such as rhenium, have been used for some time. These metals are often supported on alumina or silica-alumina. The benzene and alkyl benzene products are among the most important of those produced by the catalytic reforming process in that they have the highest octane number when used in motor fuel.
Additionally, platinum based catalysts have been used in the dehydrocylization of hexane and heptane to produce benzene and alkyl benzenes having utility in the chemical industry. Various catalysts have been suggested for use in the reforming process and include those mentioned above as well as catalysts based on the use of the Group VIII noble metals on zeolites.
Although zeolite L catalysts, usually in their hydrogen form, have been employed as catalytic dewaxing catalysts and in other applications, they are particularly useful in reforming because they decrease the amount of hydrocracking which occurs during reforming. For example, U.S. Pat. No. 4,104,320 to Bernard et al. discloses that the use of zeolite L as a support increases the selectivity of the reaction for producing aromatic products. This improvement, however, has been made at the expense of catalyst life. This catalyst may be regenerated by subjecting it to the hydrogen treatment, oxidation, oxychlorination, calcining, water treatment, and reduction of hydrogen as disclosed in French patent application 2,360,540, filed Sept. 9, 1981 to Bernard et al., or by hydrogen regeneration as is disclosed in French patent application 8,000,144 to Bernard.
Reforming/dehydrocyclization catalysts of the platinum-KL type have been disclosed in U.S. Pat. No. 4,522,856, to Tauster et al.; U.S. Pat. No. 4,595,670 to Tauster et al., U.S. Pat. No. 4,595,668 to Poeppelmeier et al., U.S. Pat. No. 4,595,669 to Fung et al. U.S. Pat. No. 4,595,669 in particular, discloses a bound reforming/dehydrocyclization catalyst comprising platinum or other Group VIII noble metal on a type L zeolite which preferably is of the potassium form. None of the publications show an extrudate having the physical and chemical properties disclosed an claimed herein, however.
Alumina is known as a binder to support type L zeolites in producing a reforming catalyst. For instance, U.S. Pat. No. 4,458,025 (to Lee et al.), U.S. Pat. No. 4,517,306 to Buss (and its divisional U.S. Pat. No. 4,447,316) make such a suggestion. Lee et al. suggests extrusion of a type L zeolite in alumina. The U.S. patent application having Serial No. 880,0B7 (to Trowbridge) suggests a process for extruding a type L zeolite catalyst using a combination of alumina derived both from boehmite and a sol. None of the patents suggests the benefits accruing from the use of the process disclosed herein.
Other disclosures which may be relevant to the invention include Gladrow et al. (U.S. Pat. No. 3,326,818) which discloses a catalyst composition made up of a crystalline aluminosilicate and a binder prepared by mixing the crystalline aluminosilicate in a minor amount of dry inorganic gel binding agent, such as alumina, containing a minor amount of a peptizing agent. The peptizing agent was said to enhance the strength of the resulting product.
The patent to Young et al. (U.S. Pat. No. 3,557,024) discloses alumina bonded catalysts which are to be used in hydrocracking processes. The catalyst composition is formed by mixing one of a number of zeolites, including zeolite L, with a binder consisting of hydrous boehmitic alumina acidified with at least 0.5 mole equivalent of a strong acid per mole of alumina. A catalyst having enhanced strength is said thus to be formed. The U.S. patent to Mitsche et al. (U.S. Pat. No. 4,046,713) suggests a method for preparing an extruded catalyst composition and acidic alumina hydrosol is admixed with a dry mixture consisting essentially of a finely divided alumina, preferably a hydrate, and a finely divided crystalline aluminosilicate such as mordenite. The resulting mixture is extruded, dried and calcined to form a catalyst said to be useful in the reforming of various naphthas.
Several patents to Johnson or Johnson et al. (U.S. Pat. Nos. 4,305,810; 4,305,811; 4,306,963; and 4,311,582) are directed to stabilized reforming catalysts which are halide promoted. Each of the catalysts is produced by employing a modified alumina support whose alumina precursor comprises at least about 75% by weight boehmite.
After the zeolite-binder mixture is formed into a shape suitable for use in a reactor, the catalytic metal must then be introduced into the zeolite.
The two generally known methods of loading Group VIII metals into a zeolite carrier using an aqueous metal solution are the impregnation and ion exchange techniques. The impregnation technique of loading a zeolite carrier generally involves loading with an amount of cationic metal solution having a volume only sufficient to fill the total pore volume of the carrier to incipient wetness (saturation). In contrast, the ion-exchange technique involves loading the metal onto a zeolite carrier with an amount of cationic solution in excess of that needed to fill the total pore volume of the carrier to incipient wetness. The excess solution is stirred with or circulated through the bed of zeolite particles. In each cases there is a rapid decrease in Group VIII metal concentration to a minimum and an equivalent increase of the non-Group VIII metal cations in solution due to the ability of the zeolite to incorporate other cations vis ion-exchange with the non-framework metal ions of the zeolite. Completion of the catalyst preparation generally involves drying and calcining the solids.
In the impregnation techniques, the solids are dried and calcined directly, whereas in the case of the ion-exchange technique the excess liquid is removed from the solids prior to drying and calcination. As is shown in U.S. Pat. No. 4,104,320, the ion exchange process may result in the production of residual acidity when, during the subsequent reduction of the Group VIII metal cations (which are at near-atomic dispersion inside the zeolite channels). hydrogen ions are formed in order to maintain charge neutrality of the zeolite structure. The acidity occurs because a large fraction of the non-framework cations that were displaced by cations during loading is removed in the discarded excess liquid prior to drying and calcination. Subsequently, when the Group VIII metal is reduced using hydrogen-containing reducing agents, these cations are no longer available to displace protons from these sites. The formation of acid sites is not a problem with the impregnation technique since the displaced ion will remain on the carrier so that when the Group VIII metal is subsequently reduced the original displaced ion can replace the proton on these sites.
U.S. Pat. No. 4,416,806 also is said to disclose the deposition of platinum on a zeolite L carrier by impregnation and exchange of ions. Also disclosed is that the carrier is immersed in a solution containing platinum for a period of time, washed and dried, and that ion exchange and impregnation may be carried out in the presence of an excess of salt of the cation of the zeolite., for instance, potassium chloride for the KL Zeolite. In U.S. Pat. No. 3,226,339 an aluminosilicate zeolite is contacted with a solution of an ionizable platinum compound and an ionizable non-platinum metal salt for a sufficient period of time said to effect uniform distribution of the platinum ion on the zeolite. While both of these patents discuss the presence of an excess of a metal salt, there is no disclosure of the particular process which is necessary to prevent acid site generation upon the drying, calcination and reduction of the zeolite carrier while avoiding an excess of metal ions in the form of a salt which could block the passage of hydrocarbons through the pores of the zeolite carrier.
In U.S. Pat. No. 3,775,502 zeolite X is mixed in an ion exchange procedure with a platinum salt and a sodium salt for several hours. Thereafter, the catalyst is washed thoroughly to remove the salt residue and then dried. Excessive water washing at this stage can cause other undesirable reactions, such as the loss of platinum from the carrier and incorporation of acidity into the carrier. Upon reduction the catalyst is given a final treatment of aqueous sodium bicarbonate salt to convert the H+ zeolite sites which have been created (also see U.S. Pat. No. 3,953,365).
In U.S. Pat. No. 4,552,B56 to Tauster et al., a process for loading platinum onto a zeolite is described. The process involves the drying of the zeolite (with or without a binder) and introducing the zeolite to a solution of the catalytic metal. The catalytic metal solution is present in such an amount that all of the solution is subsumed. The product is then dried and calcined.
In U.S. Pat. No. 4,568,656, the composition of the platinum-containing solution used to load a zeolite powder or bound zeolite substrate so to maintain a particular defined concentration of a non-platinum metal within a functionally defined range. The pH of the solution is said to be "at least 7, preferably 8.5 to 12.5".
In U.S. Pat. No. 4,608,356, to Buss et al., a process is disclosed which involves the step of contacting a zeolite L with a platinum solution for a period of less than about three hours. The impregnated zeolite is then calcined in steam.
None of the cited material teaches or suggests a method for producing a reforming/dehydrocyclization catalyst of the composition shown herein having the specific physical and chemical characteristics.